The subject amplifier system is generally directed to a system for adjustably amplifying a signal in amplitude. More specifically, the subject amplifier system is directed to such a system having a stable linearized gain which may be cleanly adjusted, or without introducing switching noise on the signal being amplified.
Adjustable gain amplifiers are widely used in the art. Examples include programmable gain amplifiers (PGAs) which typically find use in various types of analog front-end (AFE) systems. They are often employed within automatic gain control (AGC) loops to adjust the amplitude of a given signal so that it may suit the requirements of analog or digital processing circuitry further downstream. Control measures within an AGC loop cause the gain of a given PGA to be adjusted as necessary.
Numerous PGA circuit topologies are known, with each conventional topology offering notable advantages but tolerating equally notable drawbacks as a tradeoff. One commonly adopted topology is based on amplifiers whose gains are set by ratios of parameters for passive components having similar type, such as resistors or capacitors. Gain control is effected by selectively switching the passive components. This type of PGA topology is illustrated in FIG. 1, implemented as a four-level gain, amplifier with a gain controlling device formed by banks of switched resistors disposed in the negative feedback of an operational amplifier A (from the inverting output to non-inverting input, and from the non-inverting output to inverting input). The voltage gain of the resulting PGA may be adjusted by appropriately setting the switches b0, b1, b2 to control which of the feedback components R20, R21, R22, R23 remain in play and which if any are bypassed. The switch settings thereby control the resistive size of the resulting gain control device (relative to the input resistance element R1) at a particular instant.
Notable advantages of this PGA type include the intrinsic linearity and stability of its voltage gain performance, due mainly to its exclusive reliance upon passive components (resistances in the example shown) to establish the gain. Not only do the passive components engender consistent response to signals at different levels, the gain is established as a ratio of parameters for similar components. Thus, any variations in manufacturing process, supply voltage, and thermal conditions (the so-called PVT variations) which affect these components will largely cancel when the gain is established.
This PGA topology, however, comes with a significant design tradeoff in certain applications. The switching noise due to the operation of switching devices disposed directly in the signal path often proves problematic. In applications like audio systems, for instance, actuating the switches b0, b1, b2 to change the gain of the PGA circuit while processing the input signal leaves undesirable switching noise artifacts in the amplified signal. That is, the signal voltage is disturbed enough at the switching transitions to create audible noise ‘glitches’ in the output signal.
Another type of PGA known in the art employs an operational transconductance amplifier (OTA) instead of resistors or other switched passive components to control gain. An example of this PGA type is illustrated in FIG. 2, where an input OTA is coupled to an amplifier, such as the operational amplifier A shown with feedback resistances R, which provides the amplified output as a voltage. Generally, an OTA functions much as a voltage controlled current source, producing output current as a function of a difference in input voltages. Such OTA in its simplest form may be implemented as differentially paired transistor devices establishing parallel conductions paths commonly coupled to a current source.
A notable drawback of conventional OTA circuitry is its inherent nonlinearity. Incorporating a conventional OTA in a PGA thus leads to diminished linearity in the PGA's voltage gain. Conventional OTA circuitry also tends to introduce considerable gain instability to the PGA. It leaves the PGA voltage gain defined according to the parameters Gm and R which derive from components of dissimilar type and therefore do not track each other well over process, supply voltage, and temperature variations. Techniques are known in the art to remedy this, and linearize the OTA's prevailing transconductance Gm by making the parameter inversely proportional in value to a reference resistance. Even so, the problem of cleanly adjusting gain remains a challenge in practice.
In the PGA topology of FIG. 2, the PGA voltage gain may be expressed as Av=Gm·R, where Gm is the transconductance of the input OTA. The OTA may be controllable via a ctrl input signal to electronically tune the transconductance Gm. Thus, the voltage gain Av may be controlled by electronically tuning the transconductance Gm. Effectively tuning Gm without the use of any switches directly in the signal path, however, is not a trivial matter. Thus, even with the use of such PGAs employing input OTA, the problem of switching noise artifacts plaguing passive component-based PGAs of the type shown in FIG. 1 largely persists.
Another perhaps less apparent drawback of OTA-controlled PGAs is the lack of any suitable measure for maintaining a desired linear range of amplifier output during operation. For example, no suitable measures are employed to establish output linear range in a manner independent of gain variations.
Consequently, neither of the PGA types illustrated in FIGS. 1-2 nor or any other such adjustable gain amplifier known in the art provides for sufficiently reliable combination of switching noise free gain adjustment and linear gain performance that remains stable with PVT variations. Moreover, none of the adjustable gain amplifiers known in the art provides for reliable gain performance in this regard, while maintaining a substantially constant output linear range despite the variably adjusted gain.
There is therefore a need for an adjustable gain amplifier, such as a PGA, which delivers stable linear gain performance over PVT variations, and which may be adjusted in gain without imparting undue switching noise to the amplified signal. There is a further need for such adjustable gain amplifier which preserves a desired output linear range independent of gain variation.